The present invention relates to potentiometric analytical determinations. More particularly, the invention relates to a method and to an instrument for determining the pH of liquid streams.
The measurement of pH is commonly effected with a commercial pH meter using a glass electrode. This instrument measures the potential difference developed between the glass electrode and a reference electrode of constant electrical potential. The potential difference when the electrodes are removed from a standard solution and placed in a test solution is related to a difference in pH.
Immersion-type electrodes are the most commonly used electrodes. These are roughly cylindrical, and comprise a barrel or stem of inert glass which is sealed at the lower end to a tip, often hemispherical, of special pH-responsive glass. This type of electrode is used in conjunction with a separate reference electrode to complete the electrochemical cell.
Combination electrodes, which are a consolidation of the pH-responsive glass electrode and reference electrode in a single probe, are now commonly being provided in a coaxial arrangement with the reference-electrode compartment surrounding the pH sensor. Theoretical considerations favor combination electrodes which have a cylindrical symmetry and a steady state of diffusion.
The electrical circuit of a probe or pH cell is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride. The solution makes contact at one end with the test solution, and at the other end with the reference electrode. A liquid junction is formed at the area of contact between the salt bridge and the test solution.
The pH-responsive surface of the glass electrode consists of a thin membrane formed from a special glass which, after suitable conditioning, develops a surface potential that is an accurate index of the hydrogen-ion concentration of the solution in which the electrode is immersed. To permit changes in the potential of the active surface of the glass membrane to be measured, an inner reference cell of constant potential is disposed on the opposite side of the glass membrane The inner reference cell comprises a solution that has a stable concentration of hydrogen ions, e.g. a buffered solution of potassium chloride which contains counter-ions to which the inner electrode is reversible. The inner electrode commonly consists of a silver - silver chloride or a calomel electrode in a buffered solution of potassium chloride.
For the reference half-cell, a silver electrode comprising silver and silver chloride provides a highly reproducible potential in the potassium chloride bridge solution, and is the most frequently used reference electrode for process measurement, although calomel and thallium half-cells have also been used.
The above prior-art devices serve very well for measuring the pH of dilute aqueous solutions which are substantially free of suspended solids. In many industrial streams, however, this condition is not satisfied. In particular, it has been found that such devices are subject to frequent fouling, contamination, and plugging when used to monitor the pH of streams containing suspended particulate matter, high concentrations of salts such as calcium chloride, or both.